Today's Internet is mainly focused on the QoS performance for differentiated services in large bandwidths to support various multimedia services. Recently, the IP(Internet Protocol) over WDM(Wavelength Division Multiplex) architecture (the so called optical Internet) has surfaced as a next generation Internet backbone because it eliminates processing overheads which such other architectures as ATM(Asynchronous Transfer Mode) and SONET(Synchronous Optical Network) generate. Although the optical Internet provides a huge bandwidth, certain schemes have to be introduced into the architecture to furnish missing QoS and fast fault recovery capabilities in optical level.
To support QoS in the Internet, various schemes have been developed. Integrated services (Intserv) provide the ability of delivering end-to-end QoS to applications over heterogeneous networks. Existing approaches for providing the Intserv require routers to manage per-flow states and perform per-flow operations (see, e.g., R. Braden et al., “Integrated services in the internet architecture: An overview.” RFC 1633, June 1994). Meanwhile, differentiated services (Diffserv) propose scalable means to deliver IP QoS based on an aggregate traffic handling (see, e.g., S. Blake et al., “An architecture for differentiated services,” RFC 2475, December 1998). Diffserv provides traffic differentiation by classifying traffic into a few classes, with relative service priorities assigned to them.
However, the above schemes require an electronic buffer and a logical processor to implement queuing algorithms. This requirement is not readily achievable for a WDM layer since optical RAM's have not been developed yet and optical processing technology is not mature enough to support the delicate processing.
Thus, in order to guarantee the network performance effectively in optical level, it is necessary to develop a new QoS scheme which should include the following characteristics:                In order to keep data transparency, data information should be processed in all optical manner, i.e., without E/O(Electrical to Optical) and O/E(Optical to Electrical) conversions at intermediate nodes;        The new QoS mechanism should guarantee the upper levels of blocking loss rate (or blocking probability) and end-to-end delay, i.e., not CoS(Class of Service) performance but QoS performance;        The new QoS scheme should be efficiently scalable, reliable and available at WDM networks; and        From the hardware view point, the complexity should be minimized to reduce the processing time and the implementation cost.        
It is expected that current optical circuit switching would eventually evolve into optical packet switching. However, as of today, optical component and processing technologies do not support the practical optical packet switching functions. In the meantime, therefore, OBS(Optical Burst Switching) is considered as a promising solution for IP over WDM networks in the near future(see, e.g., C. Qiao and M. Yoo, “Optical burst switching(OBS)—A new paradigm for an optical internet.”, J. High Speed Network, vol.8, pp. 69–84, 1999). The OBS architecture may not require optical buffering at intermediate nodes and the adopted delay reservation scheme thereof increases the bandwidth utilization.
Along this line, an offset-time-based QoS scheme in an OBS network has been proposed(see, e.g., M. Yoo and c. Qiao, “QoS performance of optical burst switching in IP over WDM networks,” IEEE J. Selected Areas in Communications, vol. 18, no. 10, pp. 2062–2071. October 2000). This scheme uses an extra offset time instead of buffering to isolate classes of traffic. This scheme effectively works without any buffering at intermediate nodes and is very simple to implement. Although this scheme is simple and efficiently isolates the classes of traffic, it has two limitations: First, it can isolate the classes of traffic but cannot guarantee a fixed level of QoS performance: that is, it provides not QoS but CoS. Second, a burst formed at an edge switch/router may include multiple IP packets with a several megabyte size. Therefore, there may occur a significantly large pre-transmission delay in QoS offset time based OBS networks (e.g., as long as a few ms order).